The present invention is directed to an armature core and a brush-equipped DC motor using the same.
As well known, a conventional brush-equipped DC motor includes, as shown in FIGS. 12 and 13, an armature core 100 having a wire coil 101 wound therearound, a commutator 102 electrically connected to the wire coil 101 and rotatable together with the core 100, a pair of brushes 103 which enable continuous rotation of the core 100 by providing current with different polarities alternately to the coil through the rotating commutator 102.
For reducing production cost and iron loss of the core 100, as can be understood from the illustration in FIG. 12, the core 100 is in the form of a plurality of layered thin steel sheets. The reason is that a cast core or a sintered core has a high electrical resistance whereby Eddy currents become large when a variable flux passes through the core 100, thereby heating the core which causes the iron loss of the core 100 to increase and therefore a drop of the output relative to the input is considerably decreased.
In view of the foregoing circumstances, the armature core 100 had to be formed from layered thin steel sheets. However, employing layered thin steel sheets restricts the shape design of the core. Thus, for example, the cross-sectional shape which is perpendicular to the axis of the core is difficult to change in a gradual manner in the axial direction.
In detail, as can be seen from FIG. 13, each of the layered thin steel sheets of the core has a central circular portion 100a and a plurality of equi-pitched radial extensions 100b. In light of the fact that the layered thin steel sheets are planar, the axial length L of any one of the extensions 100b is constant. In addition, the cross-section of the extension 100b in the axial direction of the core becomes rectangular with four right-angle corners as shown in FIG. 15.
When winding the wire 101 around the core 100, the wire 101 is in an overlapped condition at the circular portion 100a and the resultant expansion, as shown in FIG. 14, has an axial dimension or thickness M, thereby enlarging the whole axial dimension of the core 100 by the thickness M.
In addition, as can be understood from the illustration in FIG. 15, the right-angled corners of the core cause the winding coil 101 to be spaced from the core 100. Such a spacing causes an enlargement of the axial length of the core 100. Moreover, the axial length L of any one of the extensions 100b is constant as explained previously and thus it is difficult to place an element such as a bearing or commutator closed to the core 100 in the design thereof. This also causes an enlargement of the entire axial length of the core 100.
Furthermore, as can be appreciated from FIG. 15, the closed loop of the wire 101 forms a substantial oval, whereby the shortening of the wire 101 makes it difficult to achieve an adequate performance. Thus it is not possible to increase the motor output by increasing the current flowing through the wire when a voltage is applied across the motor when the closed loop is shortened in length.
The foregoing problems relate to the fact that the shape of the core cannot be formed in an arbitrary manner.